The present invention relates to a swing control apparatus for an industrial vehicle, such as a forklift, which controls vertical swinging of the axle of the vehicle.
Conventional forklifts are typically designed in such a manner that the rear axle is supported swingable in the vertical (rolling) direction with respect to the body frame to improve the riding comfort and traveling stability of the forklift. Further, the center of gravity of a forklift varies in accordance with various factors, such as the weight of the carried load and the lifted height of the carried load. As the rear axle swings, the center of gravity of the vehicle is likely to shift in the right and left (lateral) directions. This causes the vehicle to lose lateral stability. To avoid such a shortcoming, swinging of the axle of the conventional forklift is temporarily restricted such as when the forklift carries a heavy load, when the load is lifted to a high position, or when the forklift makes a fast turn.
For example, Japanese Unexamined Patent Publication No. 58-167215 discloses a mechanism for restricting swinging of the rear axle when the carried load on the fork becomes equal to or greater than a predetermined weight and when the elevated position of the carried load becomes higher than a predetermined position. Generally, the vehicle center of gravity must be kept in a stabilizing zone, which lies within a plane defined between the contact points of the ground and the four tires of the forklift, to maintain the vehicle in a stable state.
A heavier load weight or a higher load position causes the vehicle center of gravity to shift laterally out of the stabilizing zone. Therefore, in the forklift of the aforementioned publication, the swinging of the rear axle is restricted in accordance with the load weight and the load height to guarantee stability of the vehicle.
When the forklift carries long objects such as lumber, it is difficult to hold the object so that the object's center of gravity is substantially aligned with the center of the forklift. That is, the center of gravity of the object may shift away laterally from the center of the forklift. Furthermore, the object may swing in the roll direction of the forklift when being transported. This may also laterally shift the object's center of gravity. In other words, a wider stabilizing zone is advantageous for lateral stability of the vehicle during transportation of objects.
One simple way to achieve a wider center of gravity stabilizing zone is to use double tires on each front wheel. The employment of the double tires increases the lateral width of the stabilizing zone. That is, the width of each outer wheel contributes to an increase in the width of the stabilizing zone. The lateral stability of the front portion of the vehicle is enhanced as compared with an ordinary forklift, which employs single tires for the front wheels. Since the use of double tires significantly widens the stabilizing zone, the forklift can carry long loads, such as lumber, in a stable manner.
A mast of a double-tire forklift may be tilted toward the front when carrying lumber or the like. Under such situation, the center of gravity of the vehicle moves toward the front portion of the vehicle, or to the front portion of the stabilizing zone. If swinging of the rear axle is restricted in this state, as shown in FIGS. 10 and 11, when rear wheels (51) of a forklift vehicle (50) ride over a protruding portion 53 on a ground 52, the rear portion of the vehicle 50 is elevated, while both front wheels 54 remain at the ground level. As a result, the vehicle 50 is tilted forward. This shifts the vehicle center of gravity to a further forward position and degrades the stability of the vehicle 50 in the forward direction.